Compressor diffuser with plasma actuators

ABSTRACT

There is disclosed a centrifugal compressor including an impeller rotatable about an axis and a diffuser downstream of the impeller. The diffuser has walls delimiting flow passages. Plasma actuators are positioned adjacent the walls and are operatively connectable to a source of electricity. The plasma actuators have a first electrode, a second electrode, and a dielectric layer therebetween. The first electrode is upstream of the second electrode. The first electrode is exposed to the flow passage. The second electrode is shielded from the flow passage by the dielectric layer. The plasma actuators are operable to generate an electric field through the dielectric layer. The plasma actuators are located closer to inlets of the flow passage than to outlets of the flow passages. A method of operating the compressor is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/193,363 filed on Nov. 16, 2018, the entire contents of whichare incorporated by reference herein.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, moreparticularly, to systems and methods used for improving a stall marginin centrifugal compressors of such engines.

BACKGROUND

A centrifugal compressor typically includes an impeller and a diffuserpipe assembly downstream of the impeller. The diffuser pipe assembly isconfigured for redirecting the flow of compressed air exiting theimpeller from a substantially radial direction to a substantially axialdirection relative to an axis of rotation of the impeller. In someoperating conditions, the flow within pipes of the diffuser pipeassembly may separate from walls of the pipes. This may result in areduced mass flow rate of air within the pipes, which may cause stall ofthe centrifugal compressor.

SUMMARY

In accordance with a first embodiment, there is provided a centrifugalcompressor comprising: an impeller rotatable about an axis, the impellerhaving an impeller outlet; a diffuser downstream of the impellerrelative to the flow of compressed air, the diffuser including wallsdelimiting flow passages fluidly connected at an inlet to the impelleroutlet and configured for receiving the flow of compressed air fordelivery to an outlet of the flow passage; and a plurality of plasmaactuators positioned adjacent the walls and operatively connectable to asource of electricity, the plasma actuators having a first electrode, asecond electrode, and a dielectric layer therebetween, the firstelectrode upstream of the second electrode, the first electrodes exposedto the flow passage, the second electrode shielded from the flowpassages by the dielectric layer, the plasma actuators operable togenerate an electric field through the dielectric layer, the plasmaactuators located closer to the inlet than the outlet.

In accordance with a second embodiment, there is provided a stallcontrol system for controlling stall of a centrifugal compressor of agas turbine engine, the centrifugal compressor having an impeller and adiffuser downstream of the impeller, the stall control systemcomprising: plasma actuators located in boundary layer regions invicinity of walls bounding flow passages of the diffuser; and acontroller operatively connected to the plasma actuators, the controllerhaving a processor and a computer readable medium operatively connectedto the processor, the computer readable medium having instructionsstored thereon for: detecting an impending stall situation of thecentrifugal compressor, and activating the plasma actuators forgenerating electric fields between first and second electrodes of theplasma actuators.

In accordance with a third embodiment, there is provided a method ofoperating a centrifugal compressor of a gas turbine engine, thecentrifugal compressor having an impeller and a diffuser downstream ofthe impeller, the method comprising: receiving a flow of air exiting theimpeller; separating the flow of air in sub-flows and receiving each ofthe sub-flows within a respective one of flow passages of the diffuser;and generating electric fields in boundary layer regions in vicinity ofwalls bounding the flow passages and at locations closer to the impellerthan to outlets of the flow passage thereby ionizing particles of air inthe boundary layer regions for accelerating the sub-flows in theboundary layer regions.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic cross-sectional view of a diffuser pipe assemblyof the gas turbine engine of FIG. 1 taken along a plane normal to acentral axis of the engine of FIG. 1;

FIG. 3 is an enlarged view of zone 3-3 of FIG. 2 illustrating in greaterdetail one of plasma actuators of the diffuser pipe assembly; and

FIG. 4 is a schematic view of a stall control system in accordance withone embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, acompressor section 14 for pressurizing the air, a combustor 16 in whichthe compressed air is mixed with fuel and ignited for generating anannular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases. The fan 12, the compressorsection 14, and the turbine section 18 are rotatable about a centralaxis 11 of the gas turbine engine 10.

Still referring to FIG. 1, the compressor section 14 includes a lowpressure compressor 14 a and a high pressure compressor 14 b. The lowpressure compressor 14 a may be an axial compressor that includes one ormore compressor stages each including a rotor and a stator. The highpressure compressor 14 b is a centrifugal, or radial, compressorincluding an impeller 15 and a diffuser 20′. The impeller 15 receivesair that has been compressed by the low pressure compressor 14 a alongan axial direction A relative to the central axis 11. The impeller 15includes a plurality of blades defining passages between each adjacentpair of the blades. The impeller has an inlet 15 a in which air entersin a direction generally parallel to an axial direction A relative tothe central axis 11. According to an embodiment, the inlet 15 a may liein a plane to which the central axis 11 is normal. An outlet 15 b mayhave an annular shape, such that air exits the outlet 15 b in agenerally radial direction R relative to the central axis 11. Accordingto an embodiment, the outlet 15 b is radially positioned relative to thecentral axis 11. The air within the high pressure compressor 14 b, whilebeing subjected to a pressure increase via its circulation in thepassages of the impeller 15, changes direction from being generallyaxial to being generally radial.

The diffuser 20′ of the centrifugal compressor 14 b is used forredirecting the flow of compressed air that exits the impeller 15 of thehigh pressure compressor 14 b from the radial direction R to the axialdirection A relative to the central axis 11. Many diffusers may be used,such as vane diffusers or pipe diffusers. In the present embodiment, thediffuser 20′ is of the pipe diffuser kind, but any suitable kind ofdiffuser may be used without departing from the scope of the presentdisclosure.

Referring more particularly to FIG. 2, a diffuser pipe assembly isgenerally shown at 20. The diffuser pipe assembly 20 may includediffuser pipes 22 and a diffuser case 24 to which the diffuser pipes 22are secured. The diffuser case 24 extends circumferentially around thecentral axis 11 and is configured to be secured to an engine casing 25(FIG. 1) of the gas turbine engine 10. The diffuser pipes 22 are securedto the engine casing 25 via the diffuser case 24. Each of the diffuserpipes 22 receives a portion of the flow, or a respective one ofsub-flows, of compressed air from the outlet 15 b of the impeller 15 andredirects it toward the combustor 16 (FIG. 1).

Each of the diffuser pipes 22 includes a wall 22 a that bounds a flowpassage 26. The wall 22 a may be annular in cross section. The flowpassage 26 has an inlet 26 a and an outlet 26 b. The portions of theflow each pass through throat regions 22 b of the diffuser pipes 22. Thethroat regions 22 b are defined as a location between the inlets 26 aand outlets 26 b of the flow passages 26 of the diffuser pipes 22 wherea cross-sectional area undergoes a reduction for instance to reach itssmallest area, or a smaller or reduced area. The throat regions 22 b mayor may not have an increase in cross-sectional area. Locations where thecross-sectional areas are the smallest are referred to as throats 22 b′and are located within the throat regions 22 b. Herein, across-sectional area is understood to mean an area of a cross-section ofthe diffuser pipe 22; the cross-section taken on a plane normal to acentral longitudinal axis L of the diffuser pipes 22. The plane normalto the central longitudinal axis L may perpendicularly intersect thewalls 22 a of the flow passages 26 all around the flow passages 26.

Each of the diffuser pipes 22 has a leading edge E defined by aninteraction of said diffuser pipe 22 and a circumferentially adjacentone of the diffuser pipes 22. In some cases, the throat 22 b′ may belocated at the leading edge E.

At the throats 22 b′, the velocity of the fluid circulating in thediffuser pipes 22 is maximal and the pressure is minimal. In aparticular embodiment, the velocity of the air in the diffuser pipes 22may reach Mach 1 at the throats 22 b′. As the cross-sectional areas ofthe diffuser pipes 22 increase downstream of the throats 22 b′, thevelocity of the air in the diffuser pipes 22 may increase beyond Mach 1and become supersonic. In a diffuser, the pressure ratio across thethroat 22 b′ may not be high enough for the flow to reach supersonicspeed. Consequently, the flow diffuses (slows down) and its pressureincreases downstream of the throat 22 b′.

In an embodiment, a throat region 22 b is defined as including thesection of the diffuser pipe 22 at which a decrease in cross-sectionalarea begins, to the smallest cross-sectional area (i.e., the throat 22b′). In another embodiment, a throat region is defined as including thesection of the diffuser pipe 22 at which a decrease in cross-sectionalarea begins, and the section of the diffuser pipe 22 downstream of thesmallest cross-sectional area exhibiting an increase in thecross-sectional area until the cross-sectional area is constant. Inanother embodiment, a throat region includes a portion of the diffuserpipe 22 immediately upstream of the throat 22 b′ and a portion of thediffuser pipe 22 immediately downstream of the throat 22 b′.

In a particular embodiment, a cross-section of the diffuser pipe 22 hasa circular shape or an oval shape. Other shapes may be used withoutdeparting from the scope of the present disclosure. The shape of thecross-section need not be uniform or similar from the inlet 26 a to theoutlet 26 b of the flow passages 26 of the diffuser pipe 22. A distancealong the flow passages 26 between the inlets 26 a of the flow passages26 and the throat regions 22 b′ may be less than that between the throatregions 22 b and the outlets 26 b of the flow passages 26 of thediffuser pipes 22. In a particular embodiment, a distance from theleading edges E of the flow passages 26 to the plasma actuators 30ranges from 0 to 5 times a diameter of the diffuser pipes 22 at thethroat 22 b′ downstream of the throat 22 b′. In a particular embodiment,the distance from the leading edges E of the flow passages 26 to theplasma actuators 30 ranges from 0 to 3 times the diameter of thediffuser pipes 22 at the throats 22 b′. In a particular embodiment,downstream ends of the throat regions 22 b are located from 3 to 5 timesthe diameter of the diffuser pipes 22 at the throat 22 b′ from theleading edges E of the diffuser pipes 22. In other words, the throatregions 22 b extend from the inlets 26 a of the flow passages 26 to from3 to 5 times the diameter of the flow passages 26 at the throat 22 b′where a cross-sectional area is the smallest.

In a case wherein a cross-section of the flow passages 26 is notcircular, a hydraulic diameter may be used. The hydraulic diametercorresponds to four times the area of the cross-section divided by itsperimeter.

Typically, when optimizing compressor performance and surge margin, onewould look at a conventional pipe diffuser design and compare it toanother conventional pipe diffuser design, which might improve onecharacteristic at the expense of the other (e.g. increased surge marginbut loss in efficiency). More specifically, when a compressor is pushednear its surge line, a separation of the flows, more specifically ofboundary layers in vicinity of the diffuser leading edges E or on wallsbounding the flow passages of the diffuser is increased until the flowis fully separated. This corresponds to the stall of the compressor andleads to the surge of the compressor.

In the depicted embodiment, one or more of the diffuser pipes 22 has atleast one plasma actuator 30 positioned adjacent its wall 22 a. Theplasma actuator 30 may be secured to the wall 22 a. The at least oneplasma actuator 30 may be located closer to the impeller 15 than theoutlet 26 b of the flow passage 26. The at least one plasma actuator 30may be located closer to the inlet 26 a of the flow passage 26 than theoutlet 26 b of the flow passage 26. In an embodiment, all diffuser pipes22 have one or more plasma actuators 30. The plasma actuators 30 areoperatively connectable to a source of electricity S (FIG. 3), which maybe for example, the electrical system of the gas turbine engine 10, orany external energy source. The plasma actuators 30 are configured tore-energize flow fields in boundary layers that develop near the walls22 a of the diffuser pipes 22. In the embodiment shown, the plasmaactuators 30 are located in the throat regions 22 b (e.g., such as atthe throats 22 b′) of the diffuser pipes 22.

Flow separation within the diffuser pipes 22 typically occurs anywherebetween the leading edges E of the diffuser 20 and downstream ends ofthe throat regions 22 b. In a particular embodiment, locating the plasmaactuators 30 between the leading edges E and the downstream ends of thethroat regions 22 b allows to more efficiently delay surge compared to aconfiguration where the plasma actuators are located elsewhere.

Referring now to FIG. 3, one of the plasma actuators 30 is shown ingreater detail. For the sake of clarity, only one of the plasmaactuators 30 is described herein below using the singular form. It isunderstood that the below description may apply to all of the plasmaactuators 30.

The plasma actuator 30 may include a first electrode 30 a or cathode, asecond electrode 30 b or anode, and a layer of dielectric material 30 ctherebetween. The layer of dielectric material 30 c may define a portionof the wall 22 b of the diffuser pipe 22. In a particular embodiment,the diffuser pipes 22 are suitably machined in order to receive theplasma actuators 30 while limiting impact on surface continuity in orderto avoid increasing drag within the diffuser pipes 22 compared to adiffuser pipe lacking such plasma actuators. The first and secondelectrodes 30 a, 30 b are disposed on opposite sides of the layer ofdielectric material 30 c. Stated differently, one of the electrodes 30a, 30 b is inside the diffuser pipe 22, whereas the other electrodes 30a, 30 b is outside the diffuser pipe 22. Both electrodes 30 a, 30 b areconnectable to the source of electricity S, which may be a source ofalternating current. The source of current may be batteries located inan aircraft equipped with the gas turbine engine 10. Alternatively, thesource of current may be an auxiliary power unit of the aircraft. Anysuitable source of current may be used without departing from the scopeof the present disclosure.

When both electrodes 30 a, 30 b are connected to the source ofelectricity S, an electric field is created and joins the two electrodes30 a, 30 b through the layer of dielectric material 30 c. The electricfield may ionize air molecules circulating therethrough that may thus beaccelerated.

Referring to FIGS. 2-3, the first electrode 30 a of the plasma actuator30 is located upstream of the second electrode 30 b relative to a flowdirection F of the air circulating from the inlets 26 a to the outlets26 b of the flow passages 26 of the diffuser pipes 22. The firstelectrodes 30 a are exposed to the air in the diffuser pipes 22 whereasthe second electrodes 30 b are shielded from the air circulating in theflow passages 26 by the layer of dielectric material 30 c. In thedepicted embodiment, the first and second electrodes 30 a, 30 b extendcircumferentially along a full circumference of the diffuser pipes 22.The second electrodes 30 b may be aligned with the throats 22 b′ of thediffuser pipes 22. In other words, the second electrodes 30 b of theplasma actuators 30 may overlap the throats 22 b′. In an embodiment,both electrodes 30 a and 30 b are in the throat regions 22 b of thediffuser pipes 22.

In the embodiment shown, a downstream end 30 ai of the first electrode30 a is aligned with an upstream end 30 b ₁ of the second electrode 30b. In other words, the first and second electrodes 30 a, 30 b do notoverlap. In yet other words, the electrodes 30 a, 30 b are asymmetric.

For operating the centrifugal compressor, a flow of air exiting theimpeller 15 is received. The flow of air is separated in sub-flows andeach of the sub-flows is received within a respective one of flowpassages of the diffuser. Electric fields are generated in boundarylayer regions B in vicinity of walls 22 a bounding the flow passages 26and at locations closer to the impeller 15 than to outlets 26 b of theflow passage 26 thereby ionizing particles of air in the boundary layerregions B for accelerating the sub-flows in the boundary layer regionsB.

In the depicted embodiment, each of the sub-flows is received in arespective one of the diffuser pipes 22. In the present embodiment, thesub-flows are accelerated at the throat regions 22 b within the flowpassages 26. As illustrated, the electric fields are generated byconnecting the two electrodes 30 a, 30 b of the plasma actuators 30 tothe source of electricity S.

In a particular embodiment, using plasma actuators in combination with acentrifugal compressor, which are less prone to stall/surge than axialcompressors and which tend to consume less surge margin in the lowerspeeds (e.g., when accelerating from idle to take-off) than axialcompressors, allows to meet OEI (One-Engine Inoperative) powerrequirements for turboshaft applications, which occurs at high correctedspeeds where the stall margin would be controlled by rear stages,typically a centrifugal stage. This is different than axial compressorsin which front stages, which are axial stages, control the overall surgemargin.

In a particular embodiment, the plasma actuators 30 allow the control ofthe boundary layer near the wall 22 a of the diffuser pipes 22. Thismight increase a surge margin of the compressor and might increase itsefficiency compared to a diffuser lacking such plasma actuators. Thedisclosed diffuser 20′ with plasma actuators 30 might offer more stallmargin capability at a same operating point than a diffuser lackingplasma actuators. The plasma actuators 30 might re-energize the flowfield in the boundary layer regions B and might cause the boundarylayers to re-attach the walls 22 a.

Referring now to FIG. 4, a stall control system is generally shown at100. The system 100 includes a controller 40 operatively connected tothe plasma actuators 30 via suitable links 42. The controller 40 has aprocessor 40 a and a computer readable medium 40 b operatively connectedto the processor 40 a. The computer readable medium has instructionsstored thereon for detecting an impending stall situation of thecentrifugal compressor 14 b and for activating the plasma actuators 30for generating the electric fields between the first and secondelectrodes 30 a, 30 b (FIG. 3) of the plasma actuators 30. In aparticular embodiment, the impending stall is assessed passively basedon a pre-determined limit on compressor pressure ratio, accelerationrate and/or a rate of change of the power/thrust of the engine. In aparticular embodiment, the impending stall is assessed actively via alive stall monitory systems that may use high-frequency responsepressure probes. A combination of passive and active assessment of theimpending stall may be used.

It is understood that the disclosed diffuser 20′ may be used in any typeof gas turbine engines, such as turboshafts and turboprops.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A stall control system for controlling stall of a centrifugalcompressor of a gas turbine engine, the centrifugal compressor having animpeller and a diffuser downstream of the impeller, the stall controlsystem comprising: plasma actuators located in boundary layer regions invicinity of walls bounding flow passages of the diffuser; and acontroller operatively connected to the plasma actuators, the controllerhaving a processor and a computer readable medium operatively connectedto the processor, the computer readable medium having instructionsstored thereon for: detecting an impending stall situation of thecentrifugal compressor, and activating the plasma actuators forgenerating electric fields between first and second electrodes of theplasma actuators.
 2. The stall control system of claim 1, wherein theplasma actuators are located in respective throat regions of the flowpassages of the diffuser.
 3. The stall control system of claim 2,wherein the respective throat regions extend from inlets of the flowpassages to from 3 to 5 times a diameter of the flow passages at athroat where a cross-sectional area is the smallest.
 4. The stallcontrol system of claim 1, wherein the plasma actuators have adielectric layer between the first and second electrodes.
 5. The stallcontrol system of claim 4, wherein the first electrodes are upstream ofthe second electrodes, the first electrodes exposed to the flow passage,the second electrodes shielded from the flow passages by the dielectriclayer, the plasma actuators operable to generate the electric fieldsthrough the dielectric layer.
 6. The stall control system of claim 2,wherein the throat region is defined as a section of each of the flowpassages that extends from a location where a cross-sectional areastarts to decrease to another location where the cross-sectional area isthe smallest.
 7. The stall control system of claim 2, wherein the throatregion is defined as a section of each of the flow passages that extendsfrom a location where a cross-sectional area starts to decrease toanother location downstream of where the cross-sectional area is thesmallest and where the cross-sectional area starts to be constant. 8.The stall control system of claim 1, wherein each of the flow passagesdefines a throat where a cross-sectional area of each of the flowpassages is the smallest, the second electrodes overlap the throats ofthe flow passages.
 9. The stall control system of claim 1, wherein thediffuser is a pipe diffuser assembly including a plurality of diffuserpipes circumferentially distributed around the axis, each of thediffuser pipes having a pipe inlet fluidly connected to an outlet of theimpeller and a pipe outlet fluidly connectable to a combustion chamber,the flow passages defined by the diffuser pipes.
 10. The stall controlsystem of claim 9, wherein the first and second electrodescircumferentially extend around a full circumference of each of thediffuser pipes.
 11. The stall control system of claim 9, wherein each ofthe diffuser pipes includes a throat region, the plasma actuatorslocated within the throat regions of the flow passages.
 12. The stallcontrol system of claim 9, wherein each of the diffuser pipes defines aleading edge at an inlet thereof, a distance from the leading edge tothe plasma actuators ranging from 0 to 5 times a diameter of thediffuser pipes at a throat where a cross-sectional area of the diffuserpipe is the smallest.
 13. The stall control system of claim 9, whereinthe throat region is defined as a section of each of the diffuser pipesthat extends from a location where a cross-sectional area starts todecrease to another location where the cross-sectional area is thesmallest.
 14. The stall control system of claim 9, wherein the throatregion is defined as a section of each of the diffuser pipes thatextends from a location where a cross-sectional area starts to decreaseto another location downstream of where the cross-sectional area is thesmallest and where the cross-sectional area starts to be constant. 15.The stall control system of claim 9, wherein each of the diffuser pipesdefines a throat where the cross-sectional area of each of the diffuserpipes is the smallest, the second electrodes overlap the throats.
 16. Amethod of operating a centrifugal compressor of a gas turbine engine,the centrifugal compressor having an impeller and a diffuser downstreamof the impeller, the method comprising: receiving a flow of air exitingthe impeller; separating the flow of air in sub-flows and receiving eachof the sub-flows within a respective one of flow passages of thediffuser; and generating electric fields in boundary layer regions invicinity of walls bounding the flow passages and at locations closer tothe impeller than to outlets of the flow passage thereby ionizingparticles of air in the boundary layer regions for accelerating thesub-flows in the boundary layer regions.
 17. The method of claim 16,wherein receiving the sub-flows within the respective one of the flowpassages includes receiving each of the sub-flows in a respective one ofdiffuser pipes.
 18. The method of claim 16, wherein accelerating thesub-flows at locations near the walls includes accelerating thesub-flows at throat regions within the flow passages.
 19. The method ofclaim 16, wherein generating the electric fields includes generating theelectric fields by connecting two electrodes to a source of electricity,one of the two electrodes being exposed to the flow of air, the other ofthe two electrodes being shielded from the one of the electrodes by alayer of dielectric material.